Not Applicable
The present invention is directed generally to optical transmission systems. More particularly, the invention relates to adding and/or dropping one or more optical signal wavelengths from a wavelength division multiplexed (WDM) signal in an optical communications system.
The emergence of the Internet as a means for transporting and accessing data combined with continual growth in traditional communications has greatly accelerated the need for high capacity transmission systems. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In optical transmission systems, information is typically transmitted between central processing centers, or points of presence, which are used to collect information being electronically transmitted from a number of smaller distributed locations. In these systems, it is often desirable and cost effective to distribute or collect information along the optical path between the centers without the cost of providing another central processing center. Optical add/drop (xe2x80x9cOADxe2x80x9d) devices can be used at locations in the optical system where the amount of information being transmitted and received at the location does not make it economically feasible to process all of the information being transmitted in the system.
Optical add/drop, or insert/remove, devices are generally configured to drop/remove one or more predetermined wavelengths (xe2x80x9cdrop wavelengthsxe2x80x9d) from a WDM signal entering the device and add/insert the same, or possibly different, wavelengths to the signal. For example, see U.S. Pat. Nos. 5,283,686, 5,555,118, 5,579,143, 5,600,473, 5,726,785, 5,778,118.
Many OAD devices include one or more filtering elements, i.e., Bragg gratings, Fabry-Perot filters, etc., which are used to either drop signal wavelengths for further processing or merely filter and remove the signal wavelengths from the transmission line. OAD devices that include filtering elements, or filters, allow for the reuse of the filter wavelengths to add wavelengths to the system.
In some OAD devices, the filtering elements can be turned to vary the wavelength that is being filtered. For example, the properties of a Bragg grating element can be varied to change the central reflective wavelength of the Bragg grating. In lightly populated WDM systems, sufficient bandwidth exists between signal wavelengths that the elements can be tuned to a wavelength not carrying a signal. Thus, a tunable OAD device can be provided in which the dropped or filtered wavelengths can be variably tuned according to the requirements of the system.
However, in dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d) systems, there is not sufficient bandwidth between the signal wavelengths to tune the filtering element in the OAD device to pass a wavelength without interfering with an adjacent wavelength. Therefore, many traditional OAD devices can not operate a tunable devices in DWDM systems.
This limitation of traditional OAD devices affects the flexibility of DWDM systems, particularly when planning for system upgrades or reconfigurations. For example, traditional OAD devices will have to be replaced when new or different wavelengths are to be dropped and/or added depending upon the configuration of the traditional OAD device.
One proposed solution is to place the filtering elements in one leg of an optical line switch and provide a bypass line in another leg of the switch. While bypass line switching provides the desired functionality, the use of line switches can introduce an unacceptable amount of loss into the OAD device. As such, the switched filter OAD devices can not be widely deployed in optical systems. Thus, there remains a clear need for OAD devices that provide increased flexibility and inexpensive upgrade paths for DWDM systems.
The present invention addresses the need for increasingly flexible optical transmission systems, apparatuses, and methods including add/drop and cross-connect devices for WDM systems. Optical systems of the present invention include an add/drop device and/or cross-connect device, which can reconfigurably add and drop signal wavelengths between one or more transmission paths.
The add/drop device includes a first selective element configurable to pass a first group of signal wavelengths including at least a first signal wavelength from an input path to a first add/drop path and pass continuing signal wavelengths differing from the first group of signal wavelengths to a second add/drop path. A second selective element is provided that is configurable to pass a second group of signal wavelengths including at least a second signal wavelength from the first add/drop path and continuing signal wavelengths differing from the second group of wavelengths from the second add/drop path to an output path. The second selective element also passes the second group of signal wavelengths from the second add/drop path and wavelengths differing from the second group of signal wavelengths from the first add/drop path to a drop path.
The add/drop device may also include an add path to introduce add signal wavelengths directly to the output path or via at least one of the first and second selective elements. In addition, a broadcast drop path from the input path can be provided to access all signal wavelengths entering the OAD device via the input path.
In various embodiments, the add/drop device includes first and second optical circulators. The first optical circulator includes an input port optically connected to a first add/drop port, a second add/drop port optically connected by a first add/drop path to the first add/drop port, and to an output port. The second optical circulator includes a first add/drop port optically connected by a second add/drop path to a second add/drop port, which can be optically connected to a drop port. The first selective element is optically connected between the first add/drop ports on the first and second circulators and the second selective element is optically connected between the second add/drop ports on the first and second circulators.
The reconfigurable add/drop devices of the present invention provide flexibility in adding, dropping, and cross-connecting signal wavelengths in WDM systems. The flexibility is provided without requiring that new components being installed in the system or that the system be taken offline to perform the reconfiguration.
Accordingly, the present invention addresses the aforementioned concerns by providing optical systems apparatuses, and methods having increased flexibility. These advantages and others will become apparent from the following detailed description.